Radiation-Driven Flame Spread Over Thermally-Thick Fuels in Quiescent Microgravity Environments

Microgravity experiments on flame spread over thermally-thick fuels were conducted using foam fuels to obtain low density and thermal conductivity, and thus large spread rate (Sf) over thermally-thick fuels compared to dense fuels such as PMMA. This scheme enabled meaningful results to be obtained even in 2.2 second drop tower experiments. It was found that, in contrast conventional understanding; steady spread can occur over thick fuels in quiescent microgravity environments, especially when a radiatively active diluent gas such as CO2 is employed. This is proposed to be due to radiative transfer from the flame to the fuel surface that can lead to steady spread even when conductive heat transfer from the flame to the fuel bed is negligible. Radiative effects are more significant at microgravity conditions because the flame thickness is larger and thus the volume of radiating combustion products is larger at microgravity. The effects of oxygen concentration and pressure are shown and the transition from thermally-thick to thermally-thin behavior with decreasing bed thickness is demonstrated. A simple semiquantitative model of radiation-driven flame spread rates is consistent with experimental observations. Radiative flux measurements confirm the proposed effects of diluent type and gravity level. These results are particularly noteworthy considering that the International Space Station employs CO2 fire extinguishers; our results suggest that helium may be a better extinguishing agent on both mass and mole bases at microgravity even though CO2 is much better on a mole basis at earth gravity.